mesh: Fix looking up existing friendship contexts
mesh: Fix looking up existing friendship contexts

When receiving Friend Offers we should also consider unestablished
contexts, and simply start from the beginning if that happens.

X-Original-Commit: 06bfd7830da3e4faed81becf7002dfb24df60a66

apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble